chore(package): update mocha to version 6.2.2

Closes #30

Author: greenkeeper[bot]

package.json

@@ -29,7 +29,7 @@
     "update-notifier": "^2.1.0"
   },
   "devDependencies": {
-    "mocha": "^5.0.0",
+    "mocha": "^6.2.2",
     "proxyquire": "^1.7.11",
     "sinon": "^5.0.0",
     "standard": "^10.0.1",